This invention relates to the construction of foundation columns and more particularly to the construction of foundation columns in submerged locations as for the support of wharfs, bridges and the like.
Foundation columns have long been used as support members for structures which are built upon such columns. The support provided by the foundation column is derived from the frictional forces developed between the foundation column and the ground within which the foundation column is imbedded. In a water environment, one end of the column extends into the earth but the other end must extend through the water to a height at or above the surface of the water.
The basic construction technique for a foundation column is to drill a hole and fill the hole with concrete. Where the ground does not provide adequate lateral support for the concrete during the hardening of the concrete or where the column is to extend above the ground, an outer casing must be provided to contain the concrete until the concrete has hardened. Easily removable casings, such as split-shell type casing, have long been used where access to the column could be obtained. In underwater locations, however, a casing has to be slipped from around the column in order to be removed. This could not be done without damage to the column once the concrete had hardened so these outer casing generally remained in place around the completed columns.
The cost of steel casing, however, has now become a major portion of the total cost for constructing a foundation column. Although all costs have increased, the cost of steel has risen disproportionately to the extent that the cost of the steel casings would be about one-fourth of the total job costs if the casings are left around the completed column. It is apparent that a significant competitive cost advantage can be achieved if the steel casing can be removed and re-used to construct additional foundation columns. As hereinabove explained, this re-useability feature is easily obtained at on-shore locations or at portions of a foundation column which extend into the air by simply providing hinged casing sections which can be opened after the concrete has hardened.
During construction of some on-shore piers, a casing having a generally wedge shaped plug at one end is driven into the ground to form a hole for the pier. As shown in U.S. Pat. No. 3,842,609, to Gilberd and Russian Certificate 285,621 to Brande et al, the casing is then filled with concrete and the casing is withdrawn from around the uncured concrete. A vibratory hammer is applied to the casing during removal to break the bond between the casing and the concrete. The earth which has been compacted about the casing as the casing was driven into the ground then acts to support and mold the concrete.
A more difficult problem is presented in attempting to remove the steel casing in an underwater environment. Here, the basic approach utilizes a "two-casing" technique. This basic approach is illustrated by Great Britain Patent Specification No. 732,494, which teaches an outer casing and an inner or moulding casing concentrically installed within the outer casing. The annulus between the two casings is filled with a filler material, such as sand, and the inner casing then filled with concrete. The inner casing is removed before the concrete has hardened whereupon the sand supports the concrete until the concrete has hardened. The sand is compacted by the hydrostatic head of the uncured concrete as the inner casing is withdrawn and the concrete slumps against the sand. Finally, the outside casing is removed and the filler material simply falls to the ground. The technique disclosed by said patent, however, is limited to relatively shallow depths of water wherein the surface area between the inner casing and the sand filler material does not become so large that the resulting static friction and viscous drag forces cannot be overcome by upward forces applied to the inner casing.
U.S. Pat. No. 3,316,723, discloses another variation of the "two-casing" technique. As disclosed by said patent, the inner casing is first filled partially or entirely with concrete. A filler material is then added in the annulus to a depth which does not result in excessive friction and viscous drag forces between the inner casing and the filler material. The inner casing is next moved upwardly to a height just short of the filler material level. This alternating addition of filler material and incremental extraction of the inner casing continues until the desired height for the foundation column is obtained. The inner casing is then pulled clear of the concrete column member.
One problem with this technique is the time required to alternate pouring and pulling. Additionally, if the inner casing is pulled above the level of the filler material, structural irregularities in the foundation column can occur from the resulting intermingling of filler material and concrete.
The disadvantages of the prior art are overcome by the present invention, however, and improved method are provided for constructing pier foundation columns in underwater locations.